1. Field of the Invention
This invention relates to flexible electromagnetic shielding. More specifically, the invention relates to a flexible conductive material and the inclusion of appropriately selected materials of high magnetic permeability. The resulting compound can be extruded as part of the manufacturing process for shielded cables and shielded housings for constituent cable subassemblies.
2. Description of the Prior Art
It is known in the art that sensitive electrical equipment can be affected by Electro-Magnetic Interference (EMI). It is also known in the art that there are several ways to reduce EMI. For example, EMI can be reduced by shielding the electronic equipment by enclosing it in shielded rooms and cabinets, filling any gaps therein with conductive gaskets, and also by shielding cables and cable assemblies connected to the electronic equipment with conductive outer layers.
One example of EMI shielding for rooms and cabinets is disclosed in U.S. Pat. No. 4,992,329 which describes EMI shielding in the form of a laminated sheet. The shielding effect of the sheet is provided by flakes of magnetic amorphous alloy that are deposited between layers of film prior to lamination. Another example is U.S. Pat. No. 4,965,408 which discloses flexible radiation shielding in the form of a laminated sheet. The EMI shielding effect of the sheet is accomplished by laminating a thin metal foil between layers of a flexible outer material.
Examples of conductive elastic gaskets are found in U.S. Pat. Nos. 4,948,922 and 4,937,128 which disclose conductive elastic gaskets used to fill gaps between openings in shielded rooms and cabinets. Both of these patents disclose the use of an elastic material that is electrically conductive in and of itself. Other examples are found in U.S. Pat. Nos. 4,977,295, 4,968,854 and 4,948,922 which disclose conductive elastic gaskets where the elastic material is made conductive through the inclusion of the metallic particles. U.S. Pat. No. 4,966,637 discloses a conductive elastic gasket where the requisite conductivity is provided by an outer wrapping of braided wire.
U.S. Pat. No. 4,920,233 is an example of a special purpose cable which includes a concentric form of Faraday shielding, and incidentally also in alternate embodiments includes a concentric layer of thermoplastic material loaded with ferrite powder. The purpose of that patentee's construction of cabling is to provide a high fidelity music signal transmission media which features consistent phase velocity characteristics over the frequency band of the music, by making the distributed inductance of the cable relatively large. In that patentee's embodiment of FIGS. 1 and 2, the distributed inductance is increased by disposing torroidal ferrite sleeves 28, FIGS. 1 and 2, along the cable's axial length. The function of EMI isolation is also present in that patentee's embodiments of FIGS. 1, 2 and 6 therein, but in the form of twisted metallic foils strips 34 (FIGS. 1 and 2) and 34A (FIG. 6), and a surrounding of metallic braiding (32, FIGS. 1 and 2) and 32A (FIG. 6). This results in a design requiring manufacture by multiple manufacturing steps employing multiple types of manufacturing processes, namely, the extrusion of the thermoplastic elements, and the twisting of a metallic jacket and the braiding of another jacket. This multistep and multimode manufacture in turn drives up direct cost of manufacture and also drives up needs for investment in manufacturing machinery. In another of that patentee's embodiments, FIGS. 6 and 7, the cable inductance is increased by ferrite powder in an extruded thermoplastic layer 26A (FIG. 6) and 48 (FIG. 7). These thermoplastic layers are an electrical insulation material. Thus although the ferrite particles provide inductance for purposes of that patentee's invention, the insulation characteristic of the thermoplastic matrix binder of their layers 26A (FIG. 6) and 48 (FIG. 7) would result in non-homogeneous electromagnetic leakages in the spaces between the ferrite particles, and would not produce the homogeneous conductivity in all directions ("isotropic"), as required of a Faraday shield. U.S. Pat. No. 4,960,965 discloses a cable of concentric layers where an outer layer of EMI shielding comprises conductive carbon fibers. U.S. Pat. No. 4,769,515 discloses a spirally laminated cable comprising an inner metallic core and a laminated outer layer including metallic foil designed to increase the surface area of the metallic conductor, rather than for the purpose of providing EMI protection.